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We investigated semiconductor composite materials of the AlN–50% SiC–Y3Al5O12 system, obtained by free sintering, which exhibit a high level of microwave absorption of 4.6 dB/mm. The activation energy values of electrical conductivity (Ea) for the obtained composites were calculated within the temperature range of 20–800°C. At temperatures close to room temperature (20–150 °C), Ea ranges from 0.120 to 0.075 eV and increases to 0.270–0.275 eV as the temperature rises to 350–800°C. The determined photon energy values of the electromagnetic wave, in the frequency range from 1 to 100 GHz, are from 4.13 × 10–6 to 4.13 × 10–4 eV. Using quantum electrodynamics at the atomic level, we described the process of microwave radiation absorption in the AlN–SiC semiconductor composites. Low-energy photons of electromagnetic waves incident on the surface of the AlN–SiC composite transfer their energy to the conduction electrons in the near-surface layers of the SiC phase and are absorbed by them. The conduction electrons emit photons at the same frequency of the electromagnetic wave, predominantly into the same SiC grains, within 10–8 s. This interaction results in the absorption of electromagnetic radiation, leading to the dissipation of wave energy and subsequent heating of the entire composite: initially, the SiC phase particles and, subsequently, the AlN grains. [ABSTRACT FROM AUTHOR]

The dependences of the real and imaginary parts of complex permittivity in AlN–SiC composites, with a silicon carbide content ranging from 20 to 50%, are characterized over a frequency range of 1–100 GHz. The SiC particles have average sizes of 0.8 and 2.3 μm. As the frequency increases from 1 to 100 GHz, the general trend shows a consistent decrease in the real part ε′ in inverse proportion to the frequency raised to the 1/5 power. The imaginary part ε″ first increases in direct proportion to the frequency raised to the 1/2 power when the frequency raises from 1 to 3 GHz, reaches its maximum in the 6–8 GHz range, and then monotonically decreases in inverse proportion to the frequency raised to the 1/5 power at frequencies greater than 8 GHz. Throughout the entire frequency range, ε′ and ε″ are found to be frequency-dependent. However, at frequencies above 8 GHz, the ε″ / ε′ = tgδ ratio remains constant and is not frequency-dependent. Analytical expressions are proposed for these dependences, allowing ε′ and ε″ to be calculated at any frequency within this range. To plot these dependencies, at least one experimental data point with reliable ε′ and ε″ values should be obtained, preferably between 2 and 5 GHz. This is particularly important for ε″, as it changes uniquely over the 1–10 GHz frequency range. To specify ε″ values at frequencies below 8 GHz, two methods are proposed. The first method employs geometric construction of the inscribed circle for ε″ as a function of frequency, enabling rapid determination of ε″ in the 4–8 GHz frequency range for AlN–SiC composites containing 20 to 50% SiC. The maximum deviation from true ε″ does not exceed 3%. The second method involves calculated parabolas, also inscribed in the frequency dependence of ε″. Over the 6–8 GHz range, the deviation of ε″ does not exceed 3% for SiC contents below 40%. [ABSTRACT FROM AUTHOR]

It is reported that the bulk electrical resistance of AlN–SiC composites decreases from 1.1 × 108 до 3.0 × 102 Ω cm when the content of silicon carbide increases from 20 to 55 wt %. In such composites, with a SiC content less than 50%, the porosity does not exceed 3% and, therefore, does not have a significant effect on the value of electrical resistance. The detected dependence of the electrical resistance on the SiC content has the form of a straight line inclined to the abscissa axis, if the resistance values are given on a logarithmic scale. [ABSTRACT FROM AUTHOR]

We present the results of the study of the microstructure of a ceramic composite based on aluminum nitride with different concentrations of yttrium oxide, synthesized by pressureless sintering. The thermal conductivity of the obtained materials was investigated, and the optimal amount of yttrium oxide in the composition for sintering large-sized parts used in electrotechnical devices was determined. [ABSTRACT FROM AUTHOR]

We determined the thermal conductivity of the dense structure of reaction-sintered silicon carbide, a ceramic material obtained by impregnating the SiC framework with silicon, followed by liquid-phase carbidization. The resulting SiC material has a thermal conductivity of 177 W/(m K), which is 36% of the theoretical value. [ABSTRACT FROM AUTHOR]

High-power microwave electrovacuum devices require bulk absorbers with a high absorption rate of microwave radiation. The absorption of microwave radiation by AlN–SiC composites is studied for their possible application, and it is found that high absorption rates can be achieved in the AlN–50 wt % SiC composite owing to the presence of micrometer and submicrometer SiC particles that provide complex and repeated absorption of microwave radiation. Absorption rate L is 4.6 dB/mm for disks with dimesions of ∅4.2 × 1.0 mm and 6.6 dB/mm for rings with sizes of ∅4.2 (external) × ∅1.9 (internal) × 1.0 mm. It is proposed to compare dielectric losses of the composites through the directly measured absorption rate of microwave radiation in real operating conditions of the bulk absorber in the microwave device, rather than through the value of dielectric losses ɛ'' or tan δ. The relationship between the quality factor, attenuation, and absorption of radiation is determined. It is shown that ɛ' and ɛ'' are frequency-dependent values in the frequency range of 1–40 GHz, but their ratio ɛ''/ɛ' = tan δ is a constant frequency-independent value at frequencies greater than 8 GHz. In the studied composite, the measured values of the real and imaginary parts of the complex permittivity are ɛ' = 32 and ɛ'' = 7 at a frequency of 3.3 GHz, and the calculated values are ɛ' = 19.3 and ɛ'' = 6.2 at a frequency of 34 GHz. [ABSTRACT FROM AUTHOR]

It is reported that the electrical resistance of pressureless sintered AlN–50 wt % SiC composites decreases with a simultaneous increase in their electromagnetic power absorption, when the sintering temperature is elevated from 1800 to 1950°C. An analytical expression relating the resistance decrease and the electromagnetic energy absorption increase is proposed. [ABSTRACT FROM AUTHOR]

The interrelation between the aspect ratio and the volume content of conducting particles has been theoretically established in the system "insulator–spheroidal particles" in the absence of macroscopic electrical conductivity at different relative dielectric permittivity values ε/εd. The comparison of theoretical dielectric permittivity calculations with experimental data for the AlN–24 vol % Mo composite with spherical conducting particles has enabled the correction of earlier obtained ratios ε/εd. The proposed new theoretical correlations take into account the mutual effect of conductive particles approaching the percolation threshold and make it possible to determine the dielectric permittivity of nonconductive composites by the volumetric content and morphology of conductive particles. [ABSTRACT FROM AUTHOR]

Electrophysical characteristics (electrical resistance and activation energy) of the aluminum nitride–titanium nitride ceramic composite are determined in the temperature range of 27–215°C. [ABSTRACT FROM AUTHOR]

Compositions of AlN–Y2O3–Mo and AlN–Y2O3–TiN composite materials with high thermal conductivity have been selected for bulk absorbers of microwave energy in delay systems. The thermal conductivity, electrical resistivity, and electromagnetic absorption of pressureless sintered AlN-based composites with different volume concentrations of conducting Mo and TiN particles are determined. [ABSTRACT FROM AUTHOR]

New composite materials based on AlN–Y2O3–С have been created for use as absorbers of microwave radiation in vacuum electronics. X-ray diffraction and structural studies have shown that the major phases in the structure of the composites are AlN (hexagonal and cubic), Y3Al5O12, Al3O4, and С. An increase of time of component mixing and grinding influenced structure formation, promoted an increase in the content of the Al3O4 phase, and led to enhancement of microwave radiation attenuation by the composites (from 6.5 to 7.6 dB), this being indicative of an increase in the absorption capacity of the ceramics obtained. [ABSTRACT FROM AUTHOR]

The electrophysical properties of an islet gold film on the surface of cubic boron nitride are considered. The calculated electrical resistance values are compared with the resistance of similar films on dielectrics with different heat conductivities (glass, sapphire, aluminum nitride). [ABSTRACT FROM AUTHOR]

The structure, phase composition, and temperature dependence of electrical resistivity of the thermobarically sintered cBN–NbN composite are presented. The semiconductor character of the change in resistivity from 2.5 to 1.8 Ohm cm in the range of 300–700 K is revealed. [ABSTRACT FROM AUTHOR]

Microwave attenuation in samples of a diamond micron powder based composite material structured by a nanocarbon binder vapor-deposited at subatmospheric pressure is measured at frequencies of 34.09 to 34.19 GHz in a cylindrical resonator at H mode. Return losses are determined from the measurements. Volume resistivity of the samples is measured. The potential of using diamond-containing structured composite materials as microwave radiation absorbers is demonstrated. [ABSTRACT FROM AUTHOR]

The theoretical assessment of the dielectric permittivity of the system insulator-spheroidal grains of the conductor and its comparison with the experimentally defined value in a microwave region for pressureless sintered AlN-Mo composite materials with the molybdenum concentrations of 16.6-24.0 vol % and the grain aspect ratio of 1.3-2.9 without the microscopic electrical conduction have been presented. [ABSTRACT FROM AUTHOR]

The structure and electro-physical properties of islet gold films on the surface of aluminum nitride are reported. [ABSTRACT FROM AUTHOR]

AlN-SiC-YAlO composite materials with a high absorption of microwave frequency (27-65 dB/cm) produced by pressureless sintering of mixtures consisting of AlN(2H), YO, and SiC (6H) in 46, 4, 50 wt %, respectively, have been studied. The SiC components of the mixtures were used in sizes of 1, 5, and 50 μm. It has been shown that the resistivity of the developed materials depends essentially on the materials structures: sizes of SiC inclusions, distances between them, and state of the interfaces. It has been found that the increase of the SiC inclusions sizes in the material structure from 3 to 7 μm results in the decrease of the resistivity from 10 to 90 Ω·m, and at the decrease of the SiC inclusions sizes from 3 to 0.5 μm there forms a SiC uninterrupted skeleton, which also decreases the resistivity to 210 Ω·m. Thus, composite materials that contain 50 wt % SiC (inclusions sizes of 3 μm) are the most efficient in producing absorbers of microwave radiation. Interlayers of yttrium aluminum garnet, which are located at the SiC grains boundaries, prevent the forming of AlN(2H)-SiC(6H) solid solutions and thus, make it possible to keep high dielectric characteristics of a composite material based on aluminum nitride and afford a high absorption of a microwave radiation. [ABSTRACT FROM AUTHOR]

Composite materials based on AlN-SiC have been produced by the pressureless sintering. It has been established that in the formation of the AlN-SiC-YAlO structure the yttrium-aluminum garnet locates along the boundaries of SiC and AlN grains, thus preventing the mutual dissolution of AlN-SiC. It has been found that the increase of the SiC amount from 20 to 50 wt % results in changes of the a and c parameters of the crystal lattice from 0.49821 to 0.49837 nm and from 0.5046 to 0.498 nm, respectively, and in increase of the composite absorbing ability from 8.8 to 31.4 dB/cm (at frequencies of 9.5-10.5 GHz). [ABSTRACT FROM AUTHOR]

Samples of composite with ceramic matrix of aluminum nitride have been produced by pressureless sintering of the AlN-YO powder compositions with addition of SiC-C particles of size 50 nm (agglomerates of 200 nm). The presence of a SiC solid solution in AlN has been revealed by Raman and X-ray diffraction microanalyses and measurements of microwave energy attenuation and of thermal conductivity. The basic physico-mechanical properties of the resultant AlN-(SiC-C) composites: hardness, thermal conductivity, attenuation of the microwave energy in the frequencies range from 9.5 to 10.8 GHz have been defined. [ABSTRACT FROM AUTHOR]

Wurtzitic aluminum nitride-based ceramic materials having the following high characteristics: structural ceramics with a Vickers hardness of HV = 16.8 GPa, fracture toughness KI c = 4.7−4.9 MPa·m1/2, bending strength σ = 370–430 MPa; functional ceramics for heatsinks with a thermal conductivity of 140 W/(m·K) and for microwaves absorption with dielectric constant ɛ = 23, dielectric loss tangent tan δ = 0.025, coefficient of the electromagnetic energy attenuation L to 36.3 dB/cm (at a frequency of 3 GHz) have been obtained using pressureless sintering and hot pressing. [ABSTRACT FROM AUTHOR]

Two-phase ceramic composites of the dielectric-semiconductor type having different semiconducting phase content (aluminum nitride ceramics with uniformly distributed inclusions of silicon carbide of a certain size) have been produced by pressureless sintering. These composites are characterized by Vickers hardness HV (150 N) 9.5–15.8 GPa, Palmqvist fracture toughness 3.0–4.2 MPa m0.5, bending strength 132–209 MPa, thermal conductivity 37–82 W/(m K), and by a coefficient of the microwave electromagnetic energy attenuation to 36.3 dB/cm. It has been found that as the size of silicon carbide grains in aluminum nitride-based ceramics increases, the thermal conductivity increases and microwave energy attenuation decreases, which is indicative of the decisive role of grain boundaries in scattering both phonons and microwave radiation. [ABSTRACT FROM AUTHOR]

High absorption of microwave energy in a composite with an aluminum nitride matrix and strongly elongated molybdenum grains at frequencies of 9.5-10.5 GHz has been discussed. A possibility to use such composites as bulk absorbers inside the vacuum for high-power microwave devices has been considered. [ABSTRACT FROM AUTHOR]